Within the next ten years, more than seventy million people will join the ranks of seniors. In an aging population, the articular cartilage that allows bones to smoothly move over each other wears down with time and disease, and like many tissues in the body, articular cartilage has a limited ability to heal itself.
At this time, options that help to relieve severe degenerative joint pain, or osteoarthritis, include joint replacement or fusion. As examples, approximately 200,000 total knee joint replacement operations and over 300,000 hip joint replacement operations are performed annually. While these operations are generally effective at treating the affected joint, these artificial joint implants typically only last about 10-15 years.
Chronic lower back pain also affects both work force productivity and healthcare expense. There are currently over 500,000 surgical procedures performed annually in the United States in an attempt to alleviate lower back pain even though such surgical procedures are typically only performed after the failure of more conservative therapy such as bed rest, pain and muscle relaxant medication, physical therapy or steroid injection. The source of this pain may originate from dysfunction among a plurality of anatomical structures (as described below) that are comprised in the spine, including facet joints.
To understand spinal biomechanics, and the impacts of dysfunction in therapy, it is useful to first consider the spinal anatomy. The vertebrae of the spine are conventionally subdivided into several sections. Moving from the head (cephalad) to the tailbone (caudal), the sections are cervical, thoracic, lumbar, sacral, and coccygeal.
Regardless of location, each vertebra forms two pedicles and two laminae that combine to define a spinal foramen in which the spinal cord is protected. Extending laterally from the pedicles are two transverse processes. Extending from the mid-line of the vertebra where the two laminae meet is a spinous process. These three processes serve as a connection point for ligaments and muscles.
Adjacent vertebrae are separated by an intervertebral disc and surfaces of the adjacent vertebrae form portions of two facet joints by and between the two vertebrae. Relative to a spinal segment consisting of an intermediate vertebra, an immediately adjacent cephalad vertebra, and an immediately adjacent caudal vertebra, the intermediate vertebra forms portions of four facet joints; namely, two facet joints with the cephalad vertebra, and two facet joints with the caudal vertebra.
With the above background in mind, FIGS. 1A and 1B illustrate a facet joint 20 composed of a superior articular facet 22 and an inferior articular facet 24. The superior articular facet 22 is formed by the vertebral level below the intervertebral disc (i.e., a superior articular facet projects upward from the junction of the lamina and the pedicle), whereas the inferior articular facet 24 is formed by the vertebral level above the intervertebral disc (i.e., an inferior articular facet projects downward).
On the superior articular facet 22 is a superior articular face 26, and on the inferior articular facet 24 is an inferior articular face 28. Facet joints are oriented obliquely to the sagittal plane, and the joint space itself is curved from front to back. The more posteriorly located inferior face 28 is convex, whereas the more interiorly located superior face 26 is concave.
The facet joint 20 is a synovial joint that is defined by the two opposing bony faces 26, 28 with cartilage 30 between them and a capsule 32 around the joint 20. More specifically, synovial fluid 34 is contained inside the joint 20 by the capsule 32, that is otherwise a water-tight sac of soft tissue and ligaments that fully surrounds and encloses the joint 20, and keeps the joint faces 26, 28 lubricated.
The ends of the bone articular facets 22, 24 that make up the synovial facet joint 20 are normally covered with the articular, hyaline cartilage 30 that allows the bony faces 26, 28 to glide against one another, providing the flexibility that allows the movement of vertebral bodies relative to one another.
As indicated above, there are two facet joints between each pair of vertebrae, one on each side (located posterior and lateral of the vertebral centerline), from the top and bottom of each vertebra. The joints combine with the disc space to create a three joint complex at each vertebral level, and each joint extends and overlaps neighboring vertebral facet joints, linking each other and hence the vertebra together.
The assembly of two vertebral bodies, the interposed spinal disc and the attached ligaments, muscles, and facet joints (inferior articulating processes that articulate with the superior articular processes of the next succeeding vertebra in the caudal direction) is referred to as a “spinal motion segment.” Each motion segment contributes to the overall flexibility of the spine and contributes to the overall ability of the spine to provide support for the movement of the trunk and head, and in particular, the facet joints limit torsional (twisting) motion.
When the facets of one or more vertebral bodies degenerate or otherwise become damaged such that the vertebrae no longer articulate or properly align with each other, there is a resulting loss of mobility and pain or discomfort. The functional role of the facet joints in a spinal motion segment is thus relevant to an understanding of the operative and functional advantages of the facet joint systems and methods disclosed herein, which achieve dynamic stabilization and mobility preservation without constraining motion in any plane.
As indicated above, facet joints are located on the posterior column of the spine. The context of this discussion: “anterior” refers to in front of the spinal column, and “posterior” refers to behind the column; “cephalad” means towards a patient's head (sometimes referred to as “superior”); and “caudal” (sometimes referred to as “inferior”) refers to the direction or location that is closer to the patient's feet.
Facet joints can be arthritic due to degeneration with aging, trauma, or disease (e.g., pathologies that include inflammatory, metabolic, or synovial, disorders). In addition, fractures, torn ligaments, and disc problems (e.g., dehydration or herniation) can all cause abnormal movement and alignment, putting extra stress on the surfaces of the facet joint.
The physiological response to this extra pressure is the development of osteophites, i.e., bone spurs. As the spurs form around the edges of the facet joint, the joint becomes enlarged, a condition called hypertrophy, and eventually the joint surfaces become arthritic. When the articular cartilage degenerates or wears away, the bone underneath is uncovered and rubs against bone. The joint thus becomes inflamed, swollen, and painful.
Facet joint arthritis is a significant source of neck and back pain, and is attributable to about 15-30% of persistent lower back pain complaints. Upon failure of conservative treatment for facet joint pain such as intra-articular steroids/local anesthetic injections administered under fluoroscopic guidance, some patients with chronic pain may eventually require surgical intervention for facet joint arthritis including, for example, facet rhizotomy; facet ectomony to remove the facet joint to reduce pressure on the exiting nerve root; total joint replacement or facet arthrodesis (i.e., fixation leading to fusion, where the two articulating surfaces of the joint remain immobile or grow solidly together and form a single, solid piece of bone); etc.
While these surgical procedures may alleviate back pain, many joint replacements and all fusions do not restore the normal physiological function and motion attributable to healthy anatomical form. Rather, they often significantly alter spinal biomechanics that can in turn cause or exacerbate co-existing spinal instabilities and degeneration at other spinal levels or in other joints associated with spinal motion.
There is a cause-and-effect relationship among intervertebral disc integrity, facet loads, and spinal degeneration. Specifically, the progressive loss of disc height with disc degeneration often also alters the facet joint's mechanical ability as the facet joints degenerate or dislocate, and ligaments lose elasticity and their load-carrying ability. More specifically, with disc-space narrowing, as frequently occurs with degenerative disc disease, there is an increased load in the facet joints, especially in extension, and concomitant degeneration of the facet joints and capsules.
Since the facet joint capsules are primarily loaded in flexion and in rotation, and the facet joints are the primary resistors against rotational or torsional forces (e.g., normally, the facet joints control approximately 30% of axial rotation), facet joint degeneration significantly alters spinal mobility.
The need to provide minimally invasive therapies that provide pain relief while restoring and preserving the biomechanical function of the physiological facet joints is paramount to overall spinal mobility, and to date, therapies have not adequately satisfied all of these issues, as noted below.
One therapy, facet rhizotomy, involves techniques that sever small nerves that go to the facet joint. The intent of the procedure is to stop the transmission of pain impulses along these nerves. The nerve(s) is identified using a diagnostic injection. Then, the surgeon inserts a large, hollow needle through the tissues in the low back. A radiofrequency probe is inserted through the needle, and a fluoroscope is used to guide the probe toward the nerve. The probe is slowly heated until the nerve is severed.
Another technique using pulsed radiofrequency does not actually burn the nerve, rather it is believed to stun the nerve. Yet another technique involves denervation by probe tip freezing, and still another procedure involves carefully controlled injection of botox toxin to treat muscle spasm, a protective reflex that may occur when the facets are inflamed that in turn causes the nearby muscles that parallel the spine to go into spasm.
While these procedures may provide pain relief, they do not address ongoing joint degeneration (e.g., wear on articulating surfaces), which leads to kinematic and biomechanical dysfunction that may in turn lead to transition syndrome (i.e., progression of degeneration and pain to other joints) at other levels.
While certain clinicians have advocated prosthetic total joint replacement of damaged facet joints, in practice, it is difficult to implement such a prosthesis for a variety of reasons including the variability of facet joint geometry from facet joint to facet joint, and the high level of interaction between the facet joint and the other components in the spinal column.
Moreover, joint replacement is a highly invasive and time-consuming procedure, requiring pre-preparation of joint surfaces and removal of bone, and thus there are associated risks, including blood loss and morbidity, increased anesthesia time, and increased convalescence time.
A related therapeutic treatment of the facet joint entails the provision of an artificial facet joint where the inferior facet segment, the mating superior facet segment, or both, are covered with a cap (i.e., over all, or substantially all, of the facet). One such device and related method of implantation is described in Fitz, U.S. Pat. No. Re 36,758.
While potentially viable, the capping of the facet segments has several potential disadvantages. Clinical concerns are believed to result from the disruption of the periosteum and ligamenturn teres femoris, both serving a nutrition delivery role to the femoral head, thereby leading to avascular necrosis of the bony support structure for the cap.
Another potential disadvantage of facet capping is that to accommodate the wide variability in anatomical morphology of the facets, not only between individuals, but also between levels within the spinal column, a very wide range of cap sizes and shapes is required.
Even further, implantation of the caps, such as those described in U.S. Pat. No. Re 36,758, cannot be performed on a minimally-invasive basis, and entail fairly significant preparatory steps at the implantation site (e.g., removal and/or re-shaping of bone). At least with use of caps over osteoarthritic femoral heads, the capping of articular bone ends has sometimes experienced clinical failure by mechanical loosening.
Another therapeutic treatment of the facet joint is to affix the superior articular process to the inferior articular process using a facet screw. Although the fixation therapy may alleviate symptoms associated with a degenerated facet joint, it also sacrifices some of the ability of the motion segment to move and thus sacrifices some of the ability of the spinal column to move in a natural manner.
Central and lateral spinal stenosis (joint narrowing), degenerative spondylolisthesis, and degenerative scoliosis may all result from the abnormal mechanical relationship between the anterior and posterior column structures and induce debilitating pain.
More recently, a percutaneously-implantable, facet joint stabilization device has been developed, and is described in U.S. application Ser. No. 12/238,196 (filed Sep. 25, 2008 and entitled “Method and Apparatus for Facet Joint Stabilization”), the teaching of which are incorporated herein by reference. The facet joint stabilization device generally entails a superior body and an inferior body that, when combined, form an exteriorly threaded device.
When inserted into the joint space, the inferior and superior bodies establish an engaged relationship with the corresponding inferior and superior bony faces of the facet joint anatomy, respectively, and are somewhat slidable relative to one another to facilitate near normal facet joint motion ability. While viable, areas for improvement remain, including retention, long-term functioning, and insertion techniques.
As the present disclosure contemplates accessing various vertebral elements and joints through a preferred approach that comes in from a percutaneous posterior approach, “proximal” and “distal” are defined in context of this channel of approach. Consequently, “proximal” is closer to the beginning of the channel and thus closer to the clinician, and “distal” is further from the beginning of the channel and thus more distant from the clinician.
When referencing access or delivery tools, “distal” would be the end intended for insertion into the access channel, and “proximal” refers to the opposing end, generally the end closer to the handle of the delivery tool. When referencing implants, generally “distal” would be the leading end first inserted into the joint and “proximal” refers to the trailing end, generally in an engagement with a deployment tool.
In light of the above, a need exists for additional therapies applicable to facet joints to stabilize and augment the facet joint in alleviating problems without initial resort to the more radical therapies of replacing the facet joint with a prosthesis and/or fixation of the facet joint and the inherent loss of natural movement of that motion segment.